Radiolytically generated azide radicals have been used for the formation of tyrosyl radical, TyrO˙ from tyrosine. The TyrO˙ radicals combine (2k= 4.5 × 108 dm3 mol–1 S–1, determined by pulse radiolysis) yielding bityrosine in a > 90% yield. Bityrosine formation is not suppressed in the presence of oxygen [k(TyrO˙+ O2) < 1 × 103 dm3 mol–1 S–1]. When TyrO˙ and O2˙– radicals are generated side by side in a 1:1.2 ratio, bityrosine formation is strongly suppressed and (2S,3aR,7aS)- and (2S,3aS,7aR)-3a-hydroxy-6-oxo-2,3,3a,6,7,7a-hexahydro-1H-indole-2-carboxylic acids 10 become the major final products. Their hydroperoxidic precursor is only short-lived (t½= 4.2 h at room temperature and pH 8). Upon its decay H2O2 is released. Product 10 is believed to be formed by the addition of O2˙– to the ortho- and para-position of the phenoxyl radical, followed by protonation, ring closure and hydrolysis. Based on material balance considerations an electron transfer from O2˙– to TyrO˙, although thermodynamically feasible, must play a minor role (⩽10%). The rate constant k(O2˙–+ TyrO˙) has been determined by pulse radiolysis to be 1.5 × 109 dm3 mol–1 S–1.